User Configuration > Administrative Templates > Microsoft Outlook 2016 > Security
User
HKCU\software\policies\microsoft\office\16.0\outlook\security
This policy setting allows you to determine if attachments received from senders within your organization open in Protected View. This setting only applies to Outlook accounts setup to use an Exchange server. If you enable this policy setting, attachments received from senders within your organization open in Protected View. If you disable or do not configure this policy setting, attachments received from senders within your organization do not open in Protected View.